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Time Symmetric Quantum Mechanics

  1. Mar 22, 2011 #1
    I'm just trying to get a feel for how seriously this theory is being considered these days.

    For those not familar with it, here's a somewhat okay laymans description:


    Also, mentioned in that article are some experiments done with weak measurements that supposedly give some credibility to the idea that the future can effect the present (with a boat load of caveats, however)

    I've noticed there are plenty of articles on Arxiv dealing with this as well.

    So, a couple questions:

    (1) Is this theory indeed finally being taken seriously? (Aharanov came up with it like 40 years ago!)

    (2) Since, in some sense, it is a reformulation of quantum mechanics, what are the ramifications for QFT?

    (3) Has any other experiments or work been done that validates the theory, other than the above mentioned one in the article

  2. jcsd
  3. Mar 23, 2011 #2

    A. Neumaier

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  4. Mar 23, 2011 #3


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    I think it's important to point out that the results of the experiments referenced in many of these popular articles on time-symmetric QM (for example: http://prl.aps.org/abstract/PRL/v102/i17/e173601) can be explained in terms of standard quantum mechanics involving normal flow of time.

    The math is more cumbersome (So, I've heard. I haven't worked through the calculation myself). The explanation involves some complicated interference effects in the measuring device. I am not familiar with the details of the experiment or its theoretical explanation, time symmetric or conventional. So I'll leave the explanation of the details to someone more versed in the specifics of the experiment.)

    The point is that if you want the majority of physicists to agree to accept the idea of backward causality and time symmetric QM over another interpretation of QM, you are going to need to explain something that cannot be explained by the other interpretations of QM.
    Last edited: Mar 23, 2011
  5. Mar 23, 2011 #4
    On your backward causality statement, do you mean violate causality ...

    I only ask because I beleive that's a common misconception about TSQM. It's my understanding that the way these "retroactive influences" work is that in the end they don't actually violate causality, as we typically think about it.

    It's sorta like entangled particles. On the surface and upon first introduction, it might sound like you can potentially send a signal faster than light and therefore violate causality, but when you sit down and study it in more detail, you find that nature/physics protects that from happening.

    Likewise, with TQSM. It's not like you can use TQSM to kill your grandfather ;-)
  6. Mar 23, 2011 #5
    btw, thanks for the link. I've been curious if those experiemnts could be explained in terms of regular QM. I'll read through those later.
  7. Mar 23, 2011 #6


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    I've edited my above post to try to make my point more apparent. Essentially, all I wanted to point out is that, as far as I am aware, the experiments mentioned in pop sci regarding TSQM can be explained without using a time symmetric theory.

    No Problem.
    Last edited: Mar 23, 2011
  8. Mar 23, 2011 #7
    Well, I tried to read that paper, but I don't have a subscription.

    From the abstract though, it sounded like a different experiemt then the one mentioned above?

    It would be interesting to hear how regular QM does explain the experiment I linked above. (By the way, you can also find the details on Arxiv somewhere) In that experiment, measurements made later in time make a distinct difference on earlier measurements, within the set up.

    It's hard to picture regular QM explaining this?

    But, I do agree that, if it can explain it, there is no need to abandon regular QM and switch to TQSM. TQSM needs to offer something new for it to sell itself to the general scientific community.

    Nevertheless, even if the two theories explain the exact same set of observations, and nothing more, it makes you wonder which has the correct physical interpretation of reality. What if TQSM does? ;-)
  9. Mar 23, 2011 #8


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    Of course, that is why it is considered an interpretation of QM, not a new theory. It will give the same results as the Copenhagen or MWI or any other interpretation. No interpretation predicts any experimental result which is not predicted by all other interpretations.
  10. Mar 23, 2011 #9
    Isn't the actual mathematical formulation behind behind the Copenhagen and the MWI interpretation identical though? It's just the conceptual interpretations that are different, right?

    If so, that would be a distinct difference between TQSM and these other interpretations. TQSM uses a two-vector formulation, so the math is a bit different than regular QM, in addition to having a different conceptual formulaton.
  11. Mar 23, 2011 #10


    Staff: Mentor

    You can write a given equation in an infinite number of different ways. Although they may superficially look very different, as long as the solutions are the same they are in fact the same equation.

    My understanding is that the mathematical formulation of TQSM is just another way of writing the same equations, however I am not an expert and I am just basing my statements on the claims of the authors.
  12. Mar 25, 2011 #11
    The following is from http://arxiv.org/abs/0706.1232, and it sums up pretty well what's been stated.


    "While TSQM is a new conceptual point-of-view that has predicted novel,
    verified effects which seem impossible according to standard QM, TSQM is
    in fact a re-formulation of QM. Therefore, experiments cannot prove TSQM
    over QM (or vice-versa).
    The motivation to pursue such re-formulations,
    then, depends on their usefulness. The intention of this article is to answer
    this by discussing how TSQM fulfils several criterion which any reformulation
    of QM should satisfy in order to be useful and interesting:

    • TSQM is consistent with all the predictions made by standard QM
    • TSQM has revealed new features and effects of QM that were missed
    before (§2),
    • TSQM has lead to new mathematics, simplifications in calculations,
    and stimulated discoveries in other fields (as occurred, e.g., with the
    Feynman re-formulation of QM) §3,
    • TSQM suggests generalizations of QM that could not be easily articulated
    in the old language (§4)"
  13. Apr 2, 2012 #12
    At work now, so can't read much of the linked articles. But I'll still post my quick first thoughts.

    Is time-symmetric the same thing as time-reversal invariant? I guess yes.
    Then consider for a second that classical Newtonian mechanics is also time-symmetric.
    But even back when we thought it was true, we didn't say that the future can affect the present, and we didn't find that weird in any way.

    So why would it be any different for QM? I think its perfectly normal.
  14. Apr 2, 2012 #13


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    Also, there is another time symmetric model (although they shy away from this term):


    Relational Blockworld: A Path Integral Based Interpretation of Quantum Field Theory
    W.M. Stuckey, Michael Silberstein, Timothy McDevitt
  15. May 9, 2012 #14
    Regarding the choice between TSQM and standard QM:

    (1) Fine, TSQM and QM might be empirically equivalent, but we could still appeal to broader theoretical considerations in making the choice between them

    (e.g. their coherence/inconsistency with other theories - for example, the Copenhagen instantaneous wave-function collapse is definitely inconsistent with the doctrine from SR that nothing can travel faster than the speed -- I'm not talking here about EPR, but just about any normal wave-function collapse after a measurement. Given that TSQM does not violate this SR doctrine and standard Copenhagen QM does, it looks like we have strong evidence to favour TSQM out of these two).

    (2) Given that all other physics is time-symmetric (other than 2nd law of thermodynamics), surely we would expect QM to be time-symmetric as well? Furthermore, most writers (certainly Price (1996)) on the arrow of time think that the arrow is a psychological projection (e.g. arrow of entropy causes the arrow of memory and this is projected outside the head to be the arrow of time) and that the future exists in exactly the same way as the past and present (and thus backwards causation is not major problem). The time-symmetry of the TSQM is therefore consistent with the likely nature of the arrow of time (e.g. illusory), whereas standard QM is not.
  16. May 9, 2012 #15


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    Welcome to PhysicsForums, WillRat!

    One of the advantages (as I see it) to a time symmetric interpretation is that c is respected naturally, which fits nicely with other physical considerations. Since the future and the past together form a context for an experiment, it is possible to explain entangled state statistics (including violations of Bell Inequalities).
  17. May 9, 2012 #16


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    Just on a side note: The 2nd law is, in fact, time-symmetric, too. Entropy as applied to the real world is a property of a macroscopic description (in terms of ensemble density matrices or thermodynamic variables) of a concrete microstate (e.g., a concrete many-body wave function for the system, or the positions, velocities and orientations of every single molecule in an ideal gas). It is not a property of the microstate itself.

    Since the macroscopic description averages over many degrees of freedom which the microsystem actually has, during the macro-description's time evolution information about the microstate is lost. The evolution of a ensemble density matrix is governed by the Liouville equation. But the density matrix actually increases in entropy in both directions -- i.e., if you propagate it backwards in time, its entropy also goes up because you also lose information about your system.
  18. May 9, 2012 #17


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    I'm glad you said that. It has always seemed to me that our knowledge of a system is at a relative maximum (and therefore entropy at a minimum) at some point (say the present). Therefore entropy would be higher in both time directions. Looking back in time, there are many possible pasts that could lead to this point. Similarly, there are many possible futures that evolve from this point. I am not sure that one can be said to be greater in total than the other.
  19. May 9, 2012 #18


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    Our interpretation is a form of TSQM and it does have consequences for QM and QFT (see our recent paper in Found. Phys. http://arxiv.org/abs/1108.2261). It also has serious consequences for GR (see our recent paper in Class. Quant. Grav. http://arxiv.org/abs/1110.3973). Thanks for the shout out, Dr Chinese :smile:
  20. Aug 14, 2012 #19
    I believe there is a lot of validity to the theory considering the numerous experiments being done on Entangled Particles in the 2000s.
  21. Aug 14, 2012 #20
    I believe the above quote from DrChinese says it all --- One of the advantages (as I see it) to a time symmetric interpretation is that c is respected naturally, which fits nicely with other physical considerations. Since the future and the past together form a context for an experiment, it is possible to explain entangled state statistics (including violations of Bell Inequalities).
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